1. Field of the Invention:
The present invention relates to an Otto-cycle engine and, more particularly, to an engine that uses miller cycles as well as Otto cycles, sets the expansion ratio to a high value, and increases the compression ratio, according to the operating conditions of the engine, close to the critical level at which combustion knock occurs to increase the output power and to improve the combustion.
2. Description of the Prior Art:
Known Otto-cycle engines are designed so that the compression ratio and the expansion ratio are the same. The compression ratio is limited by combustion knock occurring in full load operation. In a non-supercharged engine, the maximum value of the compression ratio is usually about 10:1. In a supercharged engine, the compression ratio cannot be increased above about 8.5:1. Therefore, the expansion ratio assumes a value of between 8.5:1 and 10:1. High-temperature, high-pressure combustion gas produced in the cylinder does not sufficiently expand and so it is not efficiently transformed into work. The gas is then exhausted as high-temperature exhaust gas. Hence, the thermal efficiency is low.
As is well known in the art, such high-temperature exhaust gas not only reduces the thermal efficiency but increases the thermal stress in the cylinder head. This thermal loading may produce cracks in the head, and heat up the exhaust valve, to a point at which its strength is reduced. Sometimes, the exhaust valve breaks or is damaged. In a supercharged engine, decreases in the expansion ratio further elevate the exhaust has temperature. As a result, the thermal stress in the engine exceeds permissible levels. In these circumstances the exhaust gas temperature is lowered by rich air-fuel mixture. This increases fuel consumption ratio.
With turbocharging, excessive thermal stress is applied to the exhaust turbine casing and other components. This adversely affects the reliability.
In an Otto-cycle engine taking in a mixture consisting of substantially equal ratios of fuel and air, it is common practice to reduce the amount of the air-fuel mixture introduced into the engine with the throttle valve in order to alleviate the load. The negative pressure created by the throttle valve increases the power loss under partial load conditions. Furthermore, the density of the compressed air-fuel mixture is reduced, which in turn leads to incomplete combustion or a decrease in the combustion speed. As a result, the indicated thermal efficiency is reduced.
Especially when recirculation of exhaust gas and combustion of a lean air-fuel mixture are effected in order to reduce the throttling loss and to reduce NOx, poor combustion takes place so increasing the emission of HC. Therefore, a limitation may be imposed on the amount of noxious emissions. When an engine is started in cold areas, it is difficult to achieve low emissions because the compression ratio is not sufficiently high.
In the conventional Otto-cycle engine where the compression ratio and the expansion ratio are set to the same value, combustion knock imposes limitations on the maximum compression ratio as described above. Consequently, the expansion ratio is also restricted. This results in various problems including a decrease in the thermal efficiency under full load and decrease in the reliability of the engine due to excessively high exhaust gas temperature. In the case of a supercharged engine where the compression ratio is set to a lower value, the exhaust gas temperature is still higher. Rich air-fuel mixture is used to lower the temperature. This increases the fuel consumption ratio. Further, the thermal burden on the exhaust gas turbine of a turbocharger is increased, and as a result, the exhaust gas turbine and the casing must be made from expensive heat-resistant alloys.
Furthermore, when the engine is under partial load, decreases in the density of the compressed air-fuel mixture occur. If recirculation of exhaust gas and lean air-fuel mixture are utilized to reduce the throttling loss and NOx, then poor combustion results. As a result, the required amount of recirculation of the exhaust gas and sufficiently lean air-fuel mixture cannot be used. It is impossible to increase the compression ratio further to elevate the compression temperature for attaining better combustion and thus enhance the thermal efficiency.